This invention relates to a stud tensioning apparatus and method and more particularly to a short stud tensioning apparatus and method for tensioning and detensioning a stud protruding a short distance above a surface.
It is often necessary to pre-tension an externally threaded stud protruding from a surface so that a nut threadably engaged thereon may be tightened against the surface or loosened after it was tightened. This operation may be accomplished using a stud tensioning device.
Stud tensioning devices have been used in many applications requiring the pre-tensioning of a stud or bolt. For example, in the nuclear power industry, stud tensioners are used as an assist for securely attaching reactor pressure vessel heads to reactor pressure vessels. A reactor pressure vessel comprises a generally cylindrical shell open at its top end and having a circumferential vessel flange integrally formed about the perimeter of the upper portion of the shell. The vessel flange includes a plurality of threaded apertures therein for anchoring the ends of a plurality of studs. Housed in the pressure vessel are a plurality of nuclear fuel assemblies submerged in a coolant such as water, which fuel assemblies produce heat that in turn generates electricity in a manner well known in the art of nuclear power production.
The open top end of the pressure vessel is covered by a generally hemispherical reactor pressure vessel head having a circumferential vessel head flange integrally formed about the perimeter of the lower portion of the vessel head. The head flange includes a plurality of holes for receiving the plurality of studs therethrough. Disposed between the head and the vessel is a compressible seal, such as double O-ring gaskets, for obtaining a seal-tight relation between the head and the vessel.
The head is mounted on the top end of the vessel so that the holes formed through the head flange matingly align with the holes formed in the vessel flange. The externally threaded studs are inserted through each hole in the head flange and screwed into the threaded holes formed in the vessel flange so that each stud is anchored in the vessel flange. Next, a nut is threadably engaged on each stud and tightened against the head flange so tht the head becomes bolted to the vessel and so that the seal compresses for obtaining a seal-tight relation between the head and the vessel.
As stated above, the nuclear fuel assemblies which are disposed in the reactor pressure vessel are submerged in a water coolant and produce heat that may raise the coolant temperature to approximately 680 degrees Fahrenheit. In a manner well known in the art, the water is used to ultimately produce steam which in turn generates electricity. However, in order to obtain a reasonable thermal efficiency, the temperature of the water should be raised the maximum extent possible prior to boiling and producing steam. Since the boiling temperature of water as a function of pressure increases as the pressure increases, nuclear power reactor vessels are hence operated at system pressures, which may be approximately 2,500 psia, substantially above atmospheric pressure.
Naturally, the relatively high pressure obtained in the reactor pressure vessel produces a correspondingly high internal pressure on the reactor pressurre vessel head. The internal pressure acting on the vessel head might lift the vessel head away from the vessel and defeat the purpose of the seal disposed between the vessel head and the vessel if the vessel head flange were not securely bolted to the vessel flange by use of the stud and nut combination referred to above. Of course, the stud and nut combination are stressed and tensioned due to the internal pressure acting on the vessel head.
In order to prevent lifting of the head and to maintain the seal-tight relation between the vessel head and the vessel, the stud and nut combination may be pre-tensioned to adequately resist the internal pressure acting against the vessel head. Pre-tensioning the stud and nut combination is accomplished by applying a force to the stud so that the stud is elongated to a predetermined pre-tension value. This pre-tension value should exceed the tension which will be exerted on the stud by the internal pressure acting on the vessel head. The nut is run down the elongated stud and tightened against the head flange so that the pre-tension axial stress in the stud and nut combination is maintained after the force applied to the stud is relaxed. This pre-tensioning operation is typically accomplished using a stud tensioning device.
The typical stud tensioning device employs a gripping means to grip the end of the stud above a nut which is threadably engaged thereon. A force is applied to the gripping means to elongate the stud while the nut is further threadably engaged about the stud and tightened against the flange surface from which the stud protrudes. The force applied to the gripping means is then relaxed and the stud and nut combination remain in pre-tension. When applied to a nuclear reactor pressure vessel, pre-tensioning the stud and nut combination maintains the seal-tight relation between the head and vessel even during reactor heat-up and cool-down. The nut can be removed from the stud by elongating the stud in the manner described above and threadably disengaging the nut from the surface from which the stud protrudes.
However, it is often necessary to pre-tension a stud that protrudes only a relatively short distance from a surface. In this instance there may not be sufficient length of the stud end exposed above the surface to allow appropriate gripping and tensioning of the stud by the tensioning device gripping means.
Although the prior art discloses stud tensioning devices, the prior art des not disclose an efficient short stud tensioning apparatus for tensioning and detensioning a stud protruding only a short distance above a surface.
One such device known in the art of tensioning a stud and nut combination is disclosed by U.S. Pat. No. 4,433,828 issued Feb. 28, 1984 in the name of Stanley R. Speigelman et al. and entitled "Reactor Vessel Stud Closure System" which is assigned to the Westinghouse Electric Corporation. This patent discloses a device for applying tension to and removing tension from a reactor vessel stud and nut combination which attaches a head of a reactor pressure vessel to a reactor vessel in a nuclear power plant. This device comprises a cylindrical gripper sleeve which encloses a split gripper whose plurality of sections are positioned around the lower end of a puller bar and the upper end of the reactor vessel stud. The plurality of sections of the split gripper, when pressed radially inward, from a cylindrically shaped structure comprising as a whole the split gripper which closes around the lower end of the puller bar and the upper end of the vessel stud. Hydraulic force is applied to the puller bar and translated to the vessel stud through the split gripper such that the vessel stud is elongated and tensioned. However, the split gripper must grip the upper end of the vessel stud above the nut engaged thereon; therefore, this device may be most effectively used to grip those studs having sufficient length exposed above the nut from which the stud protrudes for enabling the split gripper to grip the stud.
Another device known in the art for tensioning a stud is disclosed by U.S. Pat. No. 3,077,335 issued Feb. 12, 1963 in the name of John C. Singleton and entitled "Stud Tension". This patent discloses a device for applying tension to large studs in order that the nuts threaded thereon may be tightened or loosened. This device comprises a pedestal having spaced legs adapted to be lowered around each stud and nut and further comprises a cylinder disposed above the pedestal, which cylinder has a piston mounted therein. Threaded on the lower end of the piston is a drawbar having a connector disposed in the drawbar. The connector is threaded onto the stud above the nut. Pressure is applied in a manner that will cause an upward force to be exerted on the drawbar and thus on the connector causing the stud to be elongated. When sufficient force has been exerted on the stud, the nut may be tightened. However, the connector must be threaded onto the upper end of the stud above the nut engaged thereon; therefore, this device may be most effectively used to elongate those studs having sufficient length exposed above the nut from which the stud protrudes to enable the connector to be threaded onto the stud.
A stud tensioning device employing a wedging means is disclosed by U.S. Pat. No. 3,285,568 issued Nov. 15, 1966 in the name of John L. Biach and entitled "Tensioning Apparatus". This patent discloses a tensioning apparatus for precise loading of a stud or bolt having a nut engaged thereon. A puller bar of this tensioning apparatus is adapted to engage with the surfaces of the nut or equivalent structure in a releasable manner. Tensioning means which form part of the apparatus operate to exert a pull on the puller bar and, consequently, tension the bolt through or by means of the nut. Wedge means which may be in the form of a pair of washers having inclined surfaces that may be slid one upon the other to produce a wedging action are interposed between the nut and the surface through which the bolt extends for taking up the slack between the nut and the surfce. Thus, the projecting end of the bolt is provided with a nut or equivalent structure which will, in effect, serve as an extension of the bolt. Although the nut used in this device may serve as an extension of a very short bolt, the device uses wedging means to take up the slack between the nut and the surface through which the bolt extends.
Consequently, while the prior art devices provided stud tensioners that performed with some efficiency, these stud tensioners evinced difficulties in tensioning and detensioning a stud protruding a short distance above a surface.
Therefore, what is needed is an efficient short stud tensioning apparatus and method capable of tensioning and detensioning a stud protruding a short distance above a surface.